Carbon dioxide from stationary and mobile sources is believed to be a major contributor to the greenhouse gas effect. Future utilization of fossil-based fuels necessitates a technology for addressing the carbon dioxide emissions. Many of technologies aimed at sequestering carbon dioxide require a concentrated stream of pure gas to be effective. The Zero-Emission Coal Alliance (ZECA) proposed a novel power plant designed to produce pure streams of hydrogen, for energy generation, and carbon dioxide, for sequestration, from coal.
The ZECA power plant can be visualized by a series of four reactors. Coal is gasified in the presence of hydrogen in the first reactor and the resulting methane is steam-reformed into hydrogen in the subsequent reactor, where calcium oxide is used to capture carbon dioxide in situ. Calcium oxide (CaO) is regenerated in the third reactor, thus producing pure carbon dioxide. Last, hydrogen is converted to electricity in the fourth reactor employing high temperature fuel cells.
The use of calcium oxide in the ZECA power plant is not limited to carbon dioxide separation from other gaseous products. Sorption of carbon dioxide in situ allows for the equilibrium of hydrogen generating reactions to be shifted to the right. Furthermore, in presence of calcium oxide, both the reforming and the water gas shift (WGS) reactions can take place within the same reactor. The steam reforming and subsequent water gas shift reactions are as follows:Steam Reforming: CH4+H2O3H2+CO  [1];andWater Gas Shift: CO+H2OH2+CO2  [2].
Steam reforming is an endothermic process and is favored at high temperatures. If the steam reforming reaction is conducted at lower temperatures, e.g. less than 500 degrees Celsius, the exothermic water gas shift reaction becomes thermodynamically significant and the carbon monoxide (CO) levels can be further reduced with the generation of carbon dioxide (CO2). By sequestering the CO2, the reaction equilibriums are shifted towards the right and more hydrogen is generated. Therefore, the WGS reaction can be conducted at higher temperatures with more favorable kinetics.
The use of calcium oxide as a carbon dioxide sorbent is being heavily investigated for applications in both power plant designs and hydrogen production. Packed bed, moving bed, and fluidized bed reactors are proposed as suitable designs for such applications. However, these types of reactors would inevitably run into limitations presented by the physical properties of CaO and calcium carbonate (CaCO3). The reaction of CaO with CO2 is very rapid and is chemically controlled initially. Once a layer of carbonate is formed on the surface, the reaction is limited by the diffusion of CO2 through this layer. The fluidized bed design might be optimized to overcome the diffusion limits but typically at the expense of a large pressure drop across the reactor. In addition, bulk CaO is subject to loss of pore volume and sintering at high temperatures, especially when subjected to multiple cycles as necessitated by its applications.
In some systems, it is also been found there was no loss of reactivity of regenerated CaO over a period of 30 days at 629° C. when an un-pressed powder is used. However, this is primarily due to sintered CaO on the surface with reduced surface area acting as a barrier for the rest of the CaO mass and forcing a diffusion limited reaction. While it is possible to operate with powders in fluidized beds, the pressure drop associated with them is very large. In addition, working with fine powders can be problematic as a portion of it typically is entrained in the process flow and must be separated.
Adsorption enhanced reforming processes have recently been investigated. Each process utilized particulate materials (catalysts and adsorbents or combinations of both). Due to the temperature stresses induced by the reforming-regeneration cycles required, particulates would begin to fatigue thereby producing fines, which will foul process equipment and severely shorten life of the process. Additional fracturing of the particles can occur from the volume expansion from CaO to carbonate. Additionally, due to the high temperatures needed to regenerate the sorbent and recover the CO2, the materials are very likely to undergo severe sintering with a corresponding decrease in capacity with each cycle.